Revealing mistake: When the shark first attacks and bites off Jim's arm, you can see his real arm under his shirt as he's scrambling away from the water. (00:34:10)
Revealing mistake: As soon as the three survivors are coming out of the lab through the hatch underwater, they all signal each other to rise to the top. Look at Saffron Burrows, her cheeks are full of air and you can almost see the surface above her head. You can also see the exact same shot when she is about to be eaten by the last shark. And what's with that hand gesture - to cut or for the shark to stop? (01:26:30 - 01:33:10)
Revealing mistake: In the scene where L.L. Cool J. is in the flooded kitchen and the shark comes in through the door, he climbs up a shelf, then falls in the water, then climbs out again. Before he makes his way to the oven you can clearly see that under his pants he is wearing knee pads. (00:53:35)
Revealing mistake: In the shot when the helicopter is about to crash into the building, take a look at the blade. There is a wire holding the helicopter.
Revealing mistake: As one of the lab crew is showing Tomas Jane the suspended tiger shark you can see a horizontal line in the water in the background. this is the edge of the water tank used to film the exterior scenes (mentioned in DVD extras).